The present invention is directed to a process and apparatus for obtaining purified carbon monoxide from a gaseous mixture consisting primarily of hydrogen and carbon monoxide. In particular, the present invention is directed to a process and apparatus for obtaining purified carbon monoxide from a gaseous mixture consisting primarily of hydrogen and carbon monoxide utilizing an impure carbon monoxide expansion step.
In the present invention, at least part of the refrigeration for a cryogenic partial condensation carbon monoxide purification cycle is provided by an impure carbon monoxide expander. In the preferred embodiment, part of a carbon monoxide-rich process liquid from the bottom of the hydrogen stripper column is partially let down in pressure and vaporized against condensing syngas feed and/or condensing carbon monoxide-rich reflux before it is expanded in a turbine to generate refrigeration for the process. After the impure carbon monoxide stream is discharged from the turbo-expander, it is fed to a separation column where it is further purified before it is rewarmed and leaves the plant as nominally pure carbon monoxide product. Existing cryogenic carbon monoxide purification technology uses refrigeration generated by one or more of the following methods: expanding at least part of a H.sub.2 -rich stream in a turbo-expander, vaporizing externally supplied liquid nitrogen, expanding a recirculating carbon monoxide-rich heat pump fluid in a turbo-expander or Joule-Thompson expansion valve, or expanding a separate, closed-cycle heat pumping fluid in either a turbo expander or a Joule-Thompson expansion valve.
The objective of this invention is to reduce the capital costs and improve the efficiency of a cryogenic condensation cycle HyCO plant producing carbon monoxide and optionally hydrogen and/or syngas carbon monoxide-products. Existing technology uses various other methods of refrigeration that are more costly and/or less efficient.
The difficulties with existing technologies for providing refrigeration relate to the inherent nature of those refrigeration schemes. For example, H.sub.2 -rich expansion requires an expensive turbo-expander because of the low molecular weight and often high pressure of such available streams. In addition, when a high pressure hydrogen product is required, this stream must also undergo expensive and inefficient recompression as part of such a process. Externally supplied liquid nitrogen refrigeration, while extremely viable for smaller plants where the operating costs are low, is not economical for larger plants. Use of either carbon monoxide-rich, N.sub.2, or other gas mixtures in recirculating heat pump cycles is also quite expensive because of the high capital cost of the driving compressors as well as the high operating costs from the inefficiencies inherent in such recycle systems.
The present invention takes advantage of the mismatch of heating and cooling curves in most existing cryogenic partial condensation carbon monoxide purification cycles so as to improve process efficiency. Thus, there is only a negligible increase in external compression requirement with the invention. In addition, since the preferred operating pressure and flow for this invention are relatively modest and since the impure carbon monoxide stream has a moderate molecular weight, the capital cost of the associated impure carbon monoxide expander is also relatively low.
As mentioned in the first part of this section, there are several existing refrigeration schemes for cryogenic process cycles to produce carbon monoxide, hydrogen, and/or syngas.
All of these processes, including the invention described here, have several features in common. They all typically start with a crude syngas feed stream containing primarily hydrogen and carbon monoxide with lower levels of CH.sub.4, N.sub.2, Ar, and other trace hydrocarbon impurities. This syngas feed stream is cooled and partially condensed to partially separate most of the heavier components from the hydrogen. The H.sub.2 -rich stream can be washed with a condensed fluid such as CH.sub.4 to remove further impurities in what are commonly known as CH.sub.4 -wash cycles. In these wash cycles, the process refrigeration is most commonly provided by a pure carbon monoxide recycle system integrated with a carbon monoxide product compressor. In cycles without the wash step, commonly referred to as partial condensation cycles, the H.sub.2 -rich stream is commonly expanded in a turbo-expander for refrigeration (or can be simply rewarmed as is if other refrigeration is provided) before it leaves the cryogenic part of the plant as a crude hydrogen product. This crude hydrogen product is often further purified by pressure swing adsorption (PSA) and is sometimes compressed to final delivery pressure.
The remaining heavier liquid is then separated in one or more columns to remove the residual hydrogen, CH.sub.4, and optionally any other relevant impurities. The purified carbon monoxide is itself then rewarmed and typically leaves the cryogenic part of the plant as low pressure carbon monoxide product. This carbon monoxide stream is often compressed to final delivery pressure with part of the carbon monoxide stream sometimes compressed and returned to the cryogenic system to provide column reflux or as a heat pumping fluid.
There are numerous examples of this general purification scheme with various different methods of refrigeration.
One example in U.S. Pat. No. 4,217,759 to T. A. Shenoy describes a typical condensation cycle for separating synthesis gas with several vapor-liquid separators as part of the feed gas cooling to improve the process. In all of the different variations, one of the primary constants is that the refrigeration for the process is provided by expanding one of the H.sub.2 -rich overhead streams from one of the vapor-liquid separators in a turbine before it is rewarmed and exits the plant.
German Application No. DE 42 10 638 Al by R. Fabian describes a generally similar partial condensation cycle for producing high purity hydrogen and carbon monoxide. Here again, the partially condensed syngas is fed to a vapor-liquid separator and the H.sub.2 -rich overhead stream is partially rewarmed and turbo-expanded to provide refrigeration to the process.
Fabian, et al. describe a carbon monoxide recycle refrigeration system for a partial condensation cycle in U.S. Pat. No. 4,566,886. In their process, a carbon monoxide-rich liquid stream is taken after flash separation of hydrogen from the partially condensed syngas feed in a series of vapor-liquid separators. This stream is let down in pressure, vaporized, expanded in a turbo expander, and rewarmed to ambient conditions. After rewarming to ambient temperatures, this stream is combined with the low pressure syngas feed to the plant, compressed, and recycled to the cryogenic system through the syngas feed compressor. It is thus important to note that since the carbon monoxide-rich stream passing through the turbo-expander in this prior art is recycled back into the low pressure syngas feed before a recompression step, this essentially constitutes a hydrogen I carbon monoxide heat pump loop.
S. Watanabe and S. Okahayashi describe a partial condensation carbon monoxide purification system in Japanese Public Patent Disclosure Bulletin No. JP 03-267680. Here the refrigeration is provided by a hydrogen-rich stream taken from a partially condensed syngas feed stream after passing through a series of vapor-liquid separators. This hydrogen-rich stream is then expanded in an expansion turbine before it is rewarmed and discharged as hydrogen product.
J. Billy describes a methane wash type hydrogen/carbon monoxide purification cycle in U.S. Pat. No. 5,133,793 where the refrigeration is provided by an external refrigeration cycle. The preferred refrigeration cycle is a purified carbon monoxide heat pump with the carbon monoxide supplied from the purified overhead stream from the carbon monoxide-CH4 separation column.
Japanese Laid Open Patent JP 07-55331 by T. Masayuki and H. Fujita describes a process for producing a controlled composition syngas primarily for ammonia or methanol synthesis. In one embodiment, this process is refrigerated using the liquid fraction from a vapor-liquid separation of the partially condensed feed stream. This liquid fraction is partially let down in pressure across an expansion valve, vaporized in a heat exchanger and expanded further in an expansion turbine to provide refrigeration before it is rewarmed and discharged from the plant as off gas. No pure carbon monoxide or hydrogen streams are produced in this process.
U.S. Pat. No. 5,609,040 to J. Billy and F. Granier describes a partial condensation carbon monoxide purification scheme in which part of the refrigeration is provided by turbo-expanding a mixed stream of primarily methane, nitrogen, and hydrogen. This stream is generated by combining a vaporized methane-rich liquid stream from the sump of a methane removal column, a nitrogen-rich vapor from the overhead of a nitrogen removal column, and a hydrogen-rich vapor from the overhead of a feed gas vapor-liquid separator. Other embodiments turbo-expand just the nitrogen and hydrogen-rich streams and blend in an un-vaporized methane-rich liquid stream down stream of the turbo expander.
U. S. Patent No. 5,832,747 to J. D. Bassett et al describe a partial condensation process and apparatus for the production of carbon monoxide and purified syngas. Here, the refrigeration for the system is provided by turbo-expanding a hydrogen-rich stream generated by partially condensing the feed and taking the overheads from one of a series of vapor liquid separators. After turbo expansion, this hydrogen-rich stream is rewarmed and leaves the process as part of the fuel stream.
It is principally desired to provide a novel process and apparatus for obtaining purified carbon monoxide from a gaseous mixture consisting essentially of hydrogen and carbon monoxide and one or more additional impurities.
It is further desired to provide a novel process and apparatus for obtaining purified carbon monoxide from a gaseous mixture consisting essentially of hydrogen and carbon monoxide and one or more additional impurities that utilizes an impure carbon monoxide expander cycle.
It is further desired to provide a novel process and apparatus for obtaining purified carbon monoxide from a gaseous mixture consisting essentially of hydrogen and carbon monoxide and one or more additional impurities where capital costs are reduced and improved efficiency is achieved.
It is still further desired to provide a novel process and apparatus for obtaining purified carbon monoxide from a gaseous mixture consisting essentially of hydrogen and carbon monoxide and one or more additional impurities where an existing process stream pressure reduction is used for refrigeration without any additional compression required to drive the process.
It is yet further desired to provide a novel process and apparatus for obtaining purified carbon monoxide from a gaseous mixture consisting essentially of hydrogen and carbon monoxide and one or more additional impurities where use of hydrogen rich expansion is not required thereby creating a penalty due to a subsequent pressure swing adsorption system operating less effectively at the lower pressure imposed by an upstream hydrogen-rich expansion.
It is further desired to provide a novel process and apparatus for obtaining purified carbon monoxide from a gaseous mixture consisting essentially of hydrogen and carbon monoxide and one or more additional impurities where expensive additional hydrogen product compression is not required.
It is further desired to provide a novel process and apparatus for obtaining purified carbon monoxide from a gaseous mixture consisting essentially of hydrogen and carbon monoxide and one or more additional impurities where use of carbon monoxide heat pump cycles is not required where the turbo-expanded carbon monoxide must be recompressed in expensive compression equipment before it is either recycled into the cryogenic cycle or discharged from the system as product.
It is still further desired to provide a novel process and apparatus for obtaining purified carbon monoxide from a gaseous mixture consisting essentially of hydrogen and carbon monoxide and one or more additional impurities where no or more limited use of liquid nitrogen is required.
It is further desired to provide a novel process and apparatus for obtaining purified carbon monoxide from a gaseous mixture consisting essentially of hydrogen and carbon monoxide and one or more additional impurities where required capital equipment is reduced as compared with systems in the prior art.